1. Field of the Invention
The present invention pertains to current interrupting devices, and especially to those devices for power class high voltage circuits, which are capable of interrupting a fault current.
2. Description of the Related Art
Apparatus for terminating an overload or overcurrent condition in a high voltage power class network must extinguish or otherwise interrupt electrical arcs that are developed as the overload current path is broken. In many types of devices, the arc is developed between a pair of separating electrodes, one or both of which are mounted for movement away from one another. Moveable electrode arrangements are disclosed in U.S. Pat. Nos. 2,051,748; 2,223,975; 3,884,542; 4,192,572; and 4,271,339. Some of these utilize gas pressure developed by the heat of the resulting arc, to assist in the electrode's separation. Mechanical assistance is provided in U.S. Pat. No. 4,553,008 where energy stored in a coil spring assist in driving the electrodes apart. Other arrangements, such as those disclosed in U.S. Pat. Nos. 2,294,801 and 3,909,570 use gas evolving material to assist in quenching the resulting arc.
So-called fluid blast devices such as those disclosed in U.S. Pat. Nos. 2,365,509; 4,224,490; and 4,243,860 use high pressure fluid flows between the separating electrodes to quench the arc formed therebetween. At times, composite structures are associated with one or both electrodes to also provide an evolution of arc quenching gas adjacent one of the electrodes. For example, U.S. Pat. No. 2,365,509 discloses a fluid blast interrupter in which horn fiber is positioned adjacent one electrode to evolve an arc quenching gas immediately adjacent the electrode. Many current interrupting devices used today, and proposed in the past, have limited the arc quenching fluid to a gas and substantial quantities of a dielectric gas are directed at an arc, within a short time after its formation. Devices such as those disclosed in U.S. Pat. No. 4,517,425 utilize negative pressures developed in the apparatus to operate check valves controlling the flow of a dielectric gas.
Several patents provide a blast of dielectric gas to disrupt the arc formed between a pair of electrodes. The patents referred to include U.S. Pat. Nos. 2,133,938; 2,349,095; 3,544,747; 4,110,580; 4,259,556; 4,296,289; 4,418,256; 4,420,662; and 4,471,185. In addition, French Patent No. 1,422,551 also discloses a current interrupting device which uses gaseous dielectric fluids, such as sulphur hexaflouride. The last mentioned U.S. Pat. No. 4,471,185 provides a controlled transition of the gas blast from subsonic to supersonic velocities in a gas flow which is initially directed generally perpendicular to the path of the separating electrodes, but which quickly follows the direction of electrode separation.
U.S. Pat. No. 4,341,933 also discloses a gas blast in directions which, at least initially, are not parallel to the path of separating electrodes. In this patent, a pair of opposed gas streams are directed to a path of separation of opposed electrodes, so as to create a controlled flow pattern at a point in the electrode path where arcing is expected to occur during current interruption. Due to the relative dimensions of the electrodes and the cylindrical-type housing in which the electrodes travel, the gas blast travels in a direction generally parallel to that of the forming arc. Indeed, it has been observed that, in general, all of the current interrupting devices referred to above are arranged such that the forming arc and the resulting turbulent flow of the insulating medium are generally parallel to one another.
An attempt is made with devices such as those described above, to cool the outside of the arc plasma rapidly enough following a current-zero so as to avoid a continuation of arcing during a subsequent transient voltage recovery between the electrodes. However, despite precautions, many current interrupting devices in use today have been found to be susceptible to two defined failure modes. In one failure mode, the arc, i e., the plasma between the electrodes is not completely dissipated and a resulting residual plasma conductivity allows relatively small current flows which are sufficient to create substantial Joule heating during the first few dozen microseconds after a current zero.
Many of the devices in use today employ dielectric gases travelling at supersonic velocities in an effort to obtain the desired rate of cooling. A gas flow surrounding the arc and travelling axially therewith cools the interelectrode residual plasma, primarily by convection in the subsonic regions and by turbulent diffusion in the supersonic regions of gas flow. If the interrupter cools the residual plasma faster than the rate of Joule heating caused by the rising system voltage and small post-zero fault current, thermal reignition of the arc is avoided. At present, the thermal reignitions are thought to be governed by the rate of rise in transient recovery voltage, which is highest for short line faults occurring only a small distance from the interrupter.
The combination of electrons, ions and atoms surrounding the arc converts the plasma to a high temperature dielectric gas. Transient recovery voltages may produce a dielectric breakdown in the hot gas, resulting in a dielectric reignition of the arc. Dielectric reignitions at present, are thought to be governed by the peak recovery voltage of a system and have been observed to be the limiting mode of failure for faults occurring near the terminal of the interrupter.
It has been found during the course of development of the present invention that thermal and dielectric fault interrupting performance of many present day circuit breakers causes the breakers to have relatively low fault current ratings because the diameter of the residual arc at current zero increases with increasing fault current. Accordingly, more time is required to cool the larger diameter residual arc from the outside surface by turbulent diffusion. Thus, thermal reignitions are experienced at lower rates of rise of transient recovery voltage and dielectric reignitions are experienced at lower peak voltages.
Accordingly, there is a need to provide further improvements in current interrupter performance, specifically in air insulated devices.